What is formed when methanol is oxidized in the presence of $Ag_{2}O$ at an appropriate temperature?

$A$ gas is evolved when ethanol reacts with sodium. Name the gas evolved and also write the balanced chemical equation of the reaction involved.

Explain the following reactions with chemical equations:
$(i)$ Oxidation
$(ii)$ Hydrogenation
$(iii)$ Substitution

Define oxidising agents. Explain its role in the oxidation of alcohols to acids. Write the oxidation reaction of ethanol.

Select alkenes and alkynes from the following:
$C_{2}H_{4}, C_{3}H_{4}, C_{2}H_{2}, C_{4}H_{8}$

$A$ compound $C$ (molecular formula $C_2H_4O_2$) reacts with $Na$ metal to form a compound $R$ and evolves a gas which burns with a pop sound. Compound $C$ on treatment with an alcohol $A$ in the presence of an acid forms a sweet-smelling compound $S$ (molecular formula $C_3H_6O_2$). On addition of $NaOH$ to $C$,it also gives $R$ and water. $S$ on treatment with $NaOH$ solution gives back $R$ and $A$. Identify $C$,$R$,$A$,$S$ and write down the reactions involved.